At this moment the only serious alternative to Illustrator is CorelDRAW. If you want to use a vector design program just for yourself, it´s ok to use a free one like Inkscape, but if you have to prepare files to send to providers and printers, then you should choose a professional one, and the best option is CorelDRAW, where you can export and import hundreds of files even you can import and export .AI and .PSD files in CorelDRAW !!!